Hydraulic vehicle brake system

ABSTRACT

A vehicle brake system which permits both anti-lock control and different controls of independent force braking operations. In an independent force braking operation, a starting brake pressure is built up in the return circuit in the wheel brakes by way of a precharging pressure generator and the pump and is modulated in the following control phase according to the control algorithms of the selected control. Interposed between the precharging pressure generator and the return pump is a change-over valve which has three switching positions, i.e., one closed, one open, and one throttled. The change-over valve adopts its open position in the filling phase and its throttled position in the control phase. It is so configured that the switching position is achieved as a function of the initial pressure in the inlet chamber but independently of the actuating force which actuates the valve. In the absence of initial pressure, the valve gradually shifts into the open position, opening first a small and then a large cross-section. In the presence of initial pressure in the inlet chamber, the actuating force can only open the small cross-section. The actuating force is so rated that it is unable to overcome the pressure difference applied via the small cross-section which acts on the closure member of the main valve.

TECHNICAL FIELD

The present invention relates to vehicle brakes and more particularlyrelates to a hydraulic vehicle brake system which includes a brakepressure generator operable by both the vehicle driver and a prechargingpressure generator.

BACKGROUND OF THE INVENTION

Basic hydraulic vehicle brake systems are equipped with a pedal-operatedbrake pressure generator and brake circuits with wheel brakes connectedthereto. When the driver's objective is to initiate braking, the pedalis depressed, and the force applied to the pedal is transmitted to amaster brake cylinder, boosted if necessary or desired. This causesdevelopment of a pressure in the brake circuits which becomes effectivein the wheel brakes and results in a decrease of the rotating speed ofthe wheels with respect to their roll velocity. The result is that inthe tire contact area of the wheels brake forces are transmitted whichcause deceleration of the vehicle.

Wheel lock may occur in such a braking operation. Therefore, anti lockbrake systems (ABS) have been developed which modulate the wheel brakepressures under braking conditions which cause the wheel to losetraction comprises pressure modulation valves, i.e., one inlet valve andone outlet valve, and the valve switching conditions dictate whetherpressure fluid is removed from the wheel brakes for pressure reductionor pressure fluid is supplied to the wheel brakes for pressure increase.Further included is a pump which furnishes pressure fluid into the brakecircuit in order to replace the pressure fluid removed for themodulation of the wheel brake pressures. The system may have manydifferent configurations. Frequently, the so-called recirculationprinciple is used wherein the pump is configured as a non-self-primingreturn pump which returns the pressure fluid that was removed from thewheel brakes via the open outlet valve directly into the brake circuitupstream of the inlet valve. Locking of the wheels during a brakingoperation may effectively be prevented by appropriately activating thevalves and the pump.

An anti-lock vehicle brake system of this type can be improved andextended to a brake system with driving stability control or tractionslip control (DSC or TCS). In traction slip control operations, pressureis built up in the wheel brakes of the driven wheels, and the braketorque produced counteracts the drive torque so as to decrease the drivetorque to an extent that the wheels do not lose traction. Thus, spinningof the wheels when starting to drive is prevented with this method. Indriving stability control operations, a brake pressure (individual foreach wheel) is built up on the wheels of the vehicle so that the brakeforces produced generate a torque about the vertical axis of the vehiclewhich counteracts an excessive yaw rate of the vehicle.

These two types of controls and some other types of control not referredto have in common that a wheel brake pressure must be generated inindividual or in all wheel brakes without pedal application beingeffected. Therefore, these braking operations are termed as independentforce braking operations. In such braking operations, initially, thewheel brakes have to be filled with pressure fluid in a filling phase inorder to generate a starting brake pressure which can then be modulatedin a subsequent control phase in conformity with the respective controlselected.

The pump of the anti-lock control system is used to build up thestarting brake pressure. It has been shown that this pump is not in allcases capable of sufficiently quickly building up the required startbrake value alone. So-called precharging pressure generators areinstalled into the brake system which, in the filling phase, deliverpressure fluid either directly to the wheel brakes (direct conduit) orto the suction side of the pump which conducts the fluid by increasingits pressure to the wheel brakes (indirect conduit).

The precharging pressure generator can be an additional pump as has beendescribed in German patent application No. 42 13 710, for example.However, systems are also possible wherein the booster of thepedal-operated brake pressure generator is so activated that it operatesthe master brake cylinder even without pedal depression, and thehydraulic wiring of the brake circuits provides a connection between themaster brake cylinder and the suction side of the pump. A system of thistype is presented in German patent application No. 44 25 578, forexample.

A so-called change-over valve is provided in both systems between theprecharging pressure generator (pump or independently actuated masterbrake cylinder) to establish the connection at least in the controlphase but also already in the filling phase of an independentcross-sections are used to this end, as described e.g. in German patentapplication serial No. 19529272455 dated Aug. 12, 1995.

In the filling phase, these valves provide a large cross-section after ashort switching time so that the precharging pressure generator candeliver a sufficient amount of pressure fluid to the suction side of the(return) pump. However, the large cross-section suffers from thedisadvantage that the pressure fluid is delivered to the pump in anundamped manner. This causes the development of loud noise because thepressure fluid column in the supply line to the pump is either greatlyaccelerated or abruptly slowed down with the opening and closing of thesuction valve of the pump (configured as a piston pump). The pressureimpacts in the suction line of the pump which are produced especiallydue to the abrupt slowing down are transmitted as structure-borne soundand emitted as air-borne sound transmission. Hence, major noises developin an independent force braking operation which are possiblymisinterpreted by the driver and, at least, are disturbing.

Therefore, an object of the present invention is to improve upon avehicle brake system of the type described hereinabove so that anindependent force braking operation is permitted at a minimum possiblenoise level. To this effect, the present invention proposes that thereare three switching positions in the change-over valve, i.e., oneclosed, one open, and one throttled, and that the control device is soconfigured that in an independent force braking operation, thechange-over valve is caused to adopt its throttled position in thecontrol phase and its open position in the filling phase at least forone of the envisaged types of control.

However, even in controls which normally render necessary a rapid brakepressure build-up until the starting brake pressure, it will besufficient under certain circumstances to effect the pressure build-upin the filling phase only by way of the direct conduit. This can besufficient e.g. when braking on a low coefficient of friction, becausethe precharging pressure generator is then able to generate the startingbrake pressure alone. The present invention further suggests that thecontrol device includes a decision-making circuit which, on the basis ofthe data it has available, determines the starting brake pressure to beachieved in the filling phase and, when the latter pressure is lowerthan the initial pressure which can be generated by the prechargingpressure generator, maintains the change-over valve in its closedposition in the filling phase.

Another objective is to configure the change-over valve so that it isable to provide the required switching conditions in a most simplemanner. The present invention discloses configuring the valve as abistable valve which switches into the throttled or into the openposition as a function of the inlet pressure while the actuating forceremains equal.

A bistable valve of this type can be designed differently. In thepresent case, as proposed, the switching valve has two parallel arrangedcommutable passages, i.e., the passage of a pilot valve and that of amain valve, and the valve seat of the one valve is provided on the valveclosure member of the other valve, and there is a joint actuation forboth valves.

Further, it is disclosed that the valve closure member of the pilotvalve is coupled directly to the actuating tappet, on the one hand, andthe valve closure member of the main valve is coupled to the actuatingtappet by way of a lost motion clutch, on the other hand. Valves of sucha design are employed, for example, to permit quick opening of a mainvalve of large cross-section. This is because a certain pressurecompensation is produced already by opening the pilot valve with a smallcross-section so that the valve closure member of the main valve willput up only a small resistance to the actuating forces.

To achieve this object, the present invention discloses adapting thecharacteristics of a valve of this type (opening cross-sections of mainand pilot valve, lost travels, spring forces and actuating forces) sothat, when the valve is actuated under initial pressure, the actuatingforces are not sufficient to overcome the pressure forces which act uponthe valve closure member of the main valve, caused by the pressuregradient on the orifice of the pilot valve. The result is that thevalve, switched under pressure, remains in its throttled position, andthe throttling effect is determined by the switched orifice of the pilotvalve.

As has been mentioned hereinabove, it may be necessary to switch thevalve into the open position in the filling phase and into the throttledposition in the control phase.

A switching signal sequence is provided in the control device to thisend in order to initially render the open valve inactive again so thatit closes to be subsequently re-actuated. The valve is moved into thethrottled position (as explained hereinabove) when initial pressure isapplied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a hydraulic wiring diagram of the brake system ofthe present invention.

FIG. 2 is a view of a cross-section taken through the change-over valveof the brake system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The brake system includes two brake circuits I, II having an identicaldesign so that the following description applies to the first and thesecond brake circuit. The desired pressure in the brake circuits isgenerated by way of a brake pressure generator 1. The brake pressuregenerator 1 is activated by means of a pedal 6 and comprises a pneumaticbooster 2 and a tandem master brake cylinder 3 having chambers which areconnected to a supply reservoir 4 in the non-actuated initial positionof the master brake cylinder. Further, the two chambers are connected byway of brake lines to the wheel brakes 17, 18 of one brake circuitrespectively. The wheel brakes 17, 18 shown are associated either withthe wheels of an axle (black-and-white brake circuit allotment) or withthe diagonally opposite wheels on the vehicle (diagonal brake circuitallotment).

In order to modulate the wheel brake pressure which acts in the wheelbrakes, an inlet valve 11, 15 and an outlet valve 12, 16 as well as areturn pump 8 is provided for each wheel brake. Both valves are actuatedelectromagnetically, and the inlet valve 11, 15 is normally open, andthe outlet valve 12, 16 is normally closed. The inlet valve 11, 15 isarranged in the brake line, and the outlet valve 12, 16 establishes theconnection a low-pressure accumulator 13.

To reduce the wheel brake pressure, the outlet valve is [being] openedwhen the inlet valve is closed so that pressure fluid can flow off fromthe wheel brake into the low-pressure accumulator 13. To maintain thepressure, both valves will be closed. To re-increase the pressure, theinlet valve is re-opened.

The pressure fluid conducted into the low-pressure accumulator 13 isreturned into the brake circuit by means of the return pump 7. Thepressure side of the pump is connected to an intersection 21 in thejoint brake line upstream of the inlet valve 11, 15. The return pump andthe inlet and outlet valves 11, 15 and 12, 16 are actuated by a controldevice (not shown) which emits its switching signals according to adefined control algorithm.

Above all, signals from various sensors are processed in this controlalgorithm. For anti-lock control, the signals of so-called wheel sensorsare necessary which sense the rotational behavior of the wheels beingbraked. Depending on which control shall be carried out, the controldevice requires further sensor signals. For driving stability control,the signals of a yaw velocity sensor and a steering angle sensor aree.g. required. For an independent force braking operation, where a quickinitial activation of the brakes is to be effected prior to the actualpedal-operated braking (brake assistant), the signal of a pedal sensorthat senses the actuation as such and the actuating speed is required.

To be able to develop a wheel brake pressure irrespective of pedalapplication, there is provision of a change-over valve 9 and aseparating valve 10. The separating valve 10 is arranged in the brakeline upstream of the intersection 21 mentioned above, and thechange-over valve 9 is inserted into a connecting line 14 between thesuction side of the return pump 7 and the supply reservoir 4.

Because the chambers of the master brake cylinder are connected to thesupply reservoir 4 when the master brake cylinder 3 is not actuated (asexplained hereinabove), the connection is so established in thisembodiment that the connecting line 14 is connected directly to one ofthe chambers of the master brake cylinder 3.

To build up a pressure in the wheel brakes irrespective of pedalapplication which is modulated in the control phase following thefilling phase, the separating valve 10 is closed, and the change-overvalve 9 opened and the return pump 7 switched on. Pump 7 aspiratespressure fluid from the supply reservoir 4, by way of the openchange-over valve, into the intersection 21. The pressure fluid isconducted from there to the wheel brakes 17, 18 until the starting brakepressure is reached. The return flow to the supply reservoir 4 isinterrupted because the separating valve 10 is closed. Apressure-limiting valve 22 connected in parallel to the separating valve10 sets the pressure in the brake line downstream of the separatingvalve 10 to a maximum value.

The starting brake pressure is reached as soon as the respectivelyactive control (DSC, TCS) initiates reduction of the pressure in thewheel brakes. The control of the independent force braking will nowenter into the actual control phase where the pressure in the wheelbrakes is adapted to the prevailing conditions by opening and closingthe inlet and outlet valves.

It is particularly important for some independent force brakingoperations that the brake pressure is quickly built up until thestarting brake pressure, i.e., that the filling phase is passed throughquickly.

Especially in those cases where the return pump 7 is designed as anon-self-priming pump, a so-called precharging pressure generator isnecessary which is integrated in the booster 2 in this embodiment. Thebooster has a control valve 8 which is actuated by the pedal 6 andadjusts a boosting pressure as a function of the pedal pressure. Thecontrol valve is given an additional electromagnetic activation so thatthe booster can be controlled to reach its maximally attainable boostingforce irrespective of a pedal application in order to develop an initialpressure in the master brake cylinder 3 which can be put into effect inthe wheel brakes. The filling phase can be realized in different ways.In a first possibility, the change-over valve 9 remains closed and theseparating valve 10 open in the filling phase. The pressure in themaster brake cylinder is conducted to the wheel brakes by way of thebrake line exactly as in a normal braking operation. The separatingvalve 10 must be closed and the change-over valve 9 opened when theinitial pressure in the wheel brakes has been reached at the latest sothat the return pump 7 can build up a pressure in the wheel brakes whichis in excess of the initial pressure. One may proceed this way in sometypes of independent force braking operations because the rapidity ofpressure increase is not of decisive importance. This switchingvariation can also be used when one has to assume that the start brakevalue is lower than the initial pressure that can be generated by theprecharging pressure generator.

The second possibility involves opening the change-over valve 9 in thefilling phase so that a rapid pressure build-up in the wheel brakes cantake place with the assistance of the pump. The separating valve 10 canremain open at least until the initial pressure value in the wheelbrakes is reached. Signals for the separating valve 10 and thechange-over valve 9 and for the brake pressure control valve 8 are alsogenerated by the control device according to defined algorithms.

FIG. 2 shows a preferred embodiment of a change-over valve 9. Thechange-over valve includes a valve housing 32 with a sleeve 33. Amagnetic core 34 and the associated magnetic armature 36 are guided inthe sleeve one behind the other. Provided on the frontal end of themagnetic armature 36 remote from the magnetic core 34 is an actuatingtappet 37 with a valve closure member 27 which is urged against a valvemember 23 under the effect of a spring 35 compressed between themagnetic armature 36 and the magnetic core 34.

The valve itself comprises a pilot valve and a main valve and has twohydraulically parallel connected passages. Passage 41 of the pilot valveis provided in the valve member 23 and actuated directly by the valveclosure member 27. Passage 42 of the main valve is provided in the valvehousing 32 and closed by the valve member 23 which herewith forms thevalve closure member of the main valve.

The passages lead into an inlet chamber 43 which is connected to theprecharging pressure generator, i.e., to the master brake cylinder 3 inthis case, by way of transverse channels 44 in the valve housing 32 andin a valve block 31. The pressure in the inlet chamber acts upon thevalve closure member 27 and the valve member 23 in a sense closing thepassages.

The passages lead on the other side into an outlet chamber 45 which isconnected to a transverse channel 46 arranged in the valve block 31.Channel 46, in turn, is in connection to the suction side of the pump 7.The electromagnetic actuating forces, the orifice cross-section of thepilot valve, and the spring 35 are conformed to each other so that,depending on the initial pressure prevailing in the inlet chamber 42,the valve adopts either an open position, where both passages are open,or a half-open throttled position where only the pilot valve is open.When the change-over valve is actuated at the beginning of the fillingphase where the full initial pressure possible does not yet prevail inthe inlet chamber, the valve will open in two steps. This means, thatinitially the pilot valve is opened, and a certain pressure balanceoccurs already between the inlet chamber and the outlet chamber.Subsequently, upon further rise of the magnetic armature 36 after thelost travel H₂ of the lost motion clutch 24 has been overcome, byentraining the valve member 33, the main valve will open so that theentire cross-section of the valve is available.

If, however, change-over of the switching valve occurs at a later time,i.e., when the filling phase is completed and the brake system passesover into the actual control, the pilot valve will be opened, but themain valve remains closed because the full initial pressure prevails inthe inlet chamber 43.

Because the pressure fluid flows constantly to the return pump on anaverage through the now opened orifice of the pilot valve, a pressuregradient with a high pressure in the inlet chamber 43 and a low pressurein the outlet chamber 45 will be produced at this orifice. The pressuregradient acts on the effective diameter of the valve member 23, and sogreat an amount of closing force is applied that the electromagneticactuating force is no longer able to overcome the closing force. Thus,when operated under pressure, the change-over valve 7 remains in athrottled switching position.

This throttling arrangement effects damping of vibrations in thepressure fluid column upstream of the return pump, which are caused byopening and closing of the suction valve of the return pump 7 designedas a piston pump. Noise development is minimized at the same time.

The result of the properties of the valve is that the valve cannot bemoved directly from an open into a throttled position under initialpressure. It is rather possible that the valve is initially closed to bethereafter re-opened, the valve being moved to adopt its throttledposition in this switching operation (as has been explainedhereinabove). To this end, a corresponding switching sequence must beprovided in the control circuit which is triggered in those cases wherethe change-over valve 9 shall be opened completely in the filling phaseand shall be throttled in the control phase.

What is claimed is:
 1. Hydraulic vehicle brake system, comprising: abrake pressure generator operable by a driver of the vehicle, at leastone brake circuit connected to the brake pressure generator with wheelbrakes and pressure modulation valves by means of which the pressure inthe wheel brakes can be modulated according to predetermined controlalgorithms, a pump which delivers pressure fluid into the brake circuitin order to balance out the pressure fluid removed from the brakecircuits for the modulation of the wheel brake pressure, a separatingvalve located between the brake pressure generator and a connected brakecircuit, a precharging pressure generator having its pressure-sideoutlet connected to a suction side of the pump by way of an intermediarychange-over valve in order to deliver pressure fluid to the suction sideof the pump during a filling and a control phase for achieving anindependent force braking operation, and a control device for evaluatingsensor signals and for providing switching signals for the pump andvalves, wherein the change-over valve has a closed position, an openposition, and a throttled position, and wherein the control device isconfigured such that in an independent force braking operation thechange-over valve adopts its throttled position in the control phase,and wherein the control device includes at least one control algorithmwhich causes the change-over valve in the filling phase to adopt itsopen position.
 2. Brake system as claimed in claim 1, wherein thecontrol device includes a decision-making circuit which determines thestarting brake pressure to be achieved in the filling phase andmaintains the change-over valve in its closed position in the fillingphase, when the starting brake pressure is lower than the pressureprovided by the precharging pressure generator.
 3. Vehicle brake systemas claimed in claim 1, wherein the change-over valve is configured as abistable valve which switches into the throttled or into the openposition as a function of the inlet pressure, with the actuating forcebeing equal.
 4. Vehicle brake system as claimed in claim 3, wherein thebistable valve includes a pilot valve and a main valve connected inparallel thereby forming a parallel pair of valves, wherein a valve seatof one valve of said parallel valve pair is provided on a valve closuremember of the other valve of said parallel valve pair, and wherein bothvalves in said parallel pair of valves have a respectively associatedvalve actuating member and wherein both valve closure members areoperable by means of one single actuating tappet.
 5. Vehicle brakesystem as claimed in claim 4, wherein the valve closure member of themain valve is coupled to the actuating tappet by way of a lost motionclutch.
 6. Vehicle brake system as claimed in claim 5, wherein thebistable valve is closed in its non-actuated position, and in that thevalve closure member of the main valve has an effective surface which isadjacent to the inlet chamber of the valve so that the actuating forceis not sufficient to switch the main valve when a defined pressuredifference acts upon the valve-closure member.
 7. Brake system asclaimed in claim 1, wherein the control device generates a switchingsignal sequence to switch the change over valve from the open into thethrottled position, and the electromagnetic actuation of the change-overvalve is initially rendered deenergized so that the change-over valvecloses, and in that subsequently a switching signal is generated so thatthe change-over valve is moved to adopt the throttled position when aninitial pressure prevails in the inlet chamber.